In cellular mobile radio systems it is fundamental that a mobile with an established connection on a radio channel shall be able to maintain the established connection when moving from one cell served by one base station to another cell served by another base station. It is also highly desirous that a mobile with an established connection on a radio channel shall be able to maintain the established connection when moving within the same cell and the radio channel used is subject to increased interference. The process by which a mobile can be able to maintain an established connection when moving in cells in a cellular radio system is generally called handoff.
In general a radio communication is only possible when the desired information carrying radio signals have a sufficient signal strength at the receiver and are sufficiently strong in relation to noise and interfering radio signals at the receiver. The minimum strength of course depends on the particular features of the system, e.g. the kind of modulation and receiver used. In order to make sure an established connection may continue on an intended radio channel between a mobile and an intended base station a handoff process comprises some kind of measurements on radio signals at the intended base and/or mobile station.
The first cellular mobile radio systems in public user were analog systems for speech or other analog information. The systems comprised multiple radio channels for transmitting analog information between base and mobile stations by transmitting analogously modulated radio signals. In general the first cellular mobile radio systems had comparably large cells. In general the signal measurements in the handoff process in such systems were performed by the base stations. One such system is the Nordic Mobile Telephone system NMT 450. Another known cellular mobile radio system of particular interest as a background to the present invention is the AMPS mobile radio system in U.S.A.
Recently digital cellular mobile radio systems for public use have been designed. Digital cellular mobile radio systems comprise digital radio channels for transmitting digital or digitized analog information between base and mobile stations by transmitting digitally modulated radio signals. Digital cellular mobile radio systems may offer substantial advantages over analog cellular mobile radio systems. To achieve these advantages there are certain demands. In particular channel supervision must be frequent and the handoff process must be rapid and be allowed to be frequent in relation to old analog systems.
One digital mobile radio system intended to be a common system for many european countries is the GSM system. In european countries already having an analog cellular mobile system the new digital GSM system is intended to be introduced as a new system independent of any old existing analog system. The GSM system base and mobile stations has not been designed to be compatible with old existing systems but to give optimum performance in various aspects all by itself. Accordingly there has been a comparatively great freedom of choice in technical matters when designing the GSM system. In particular this is true with the handoff method in the GSM system. In the GSM system measurements for the purpose of handoff are done both by base and mobile station.
In contrast to introducing a new independent digital cellular mobile radio system like the GSM system in an area with an existing analog cellular system it has been proposed to introduce a digital cellular mobile radio system designed for cooperation with the existing analog cellular mobile radio system. To obtain digital radio channels within the frequency band allotted to cellular mobile radio systems it is proposed to withdraw a number of radio channels allotted to present analog mobile radio systems and use them in digital cellular mobile radio system. Due to the proposed design of the digital mobile radio system three or possibly six digital radio channels may use in time division multiplex the frequency band of one previous analog radio channel. Accordingly replacing some analog channels by multiplex digital radio channels in time division may increase the total number of radio channels in the joint analog and digital systems.
The intention is to gradually introduce the digital system and with time gradually increase the number of digital channels and decrease the number of analog channels in the coexisting cellular systems. Analog mobiles already in use shall be able to continue to use remaining analog channels. New digital mobiles shall be able to use new digital channels. New dual-mode mobiles shall be able to use both remaining analog channels and new digital channels. In such a system the handoff process for digital radio channels can not simply be selected to be identical with any handoff process for any digital system, e.g. the GSM system. The handoff process for digital channels must not interfere with the handoff process of the existing analog system and must not require amendments of signals in the analog system. There are special advantages if some control channels or signals from base stations may be used for various purposes in both the analog and the digital system. The present invention aims at such a handoff method. The invention also aims at a base station and a mobile station comprising means for communication and performing such handover for digital channels.